In recent years, cameras are embedded in wide variety of products. In general, the cameras are mounted, as camera modules, in electronic devices such as a mobile phone and a laptop computer. In a case where the camera modules are mounted in such small-sized electronic devices, it is strongly required to downsize the cameras themselves or to reduce electric power consumed by the cameras themselves.
Such camera modules include, in some cases, a focus adjusting mechanism which changes a focal distance or a focal position of an optical system. Conventional focus adjusting mechanisms often employ a method of adjusting a focus by moving a lens. As a method of moving a lens, there are known (i) a voice coil motor method in which a lens is moved up and down by use of a magnetic force or (ii) a method in which a lens is moved by driving a piezoelectric element.
Those methods require a lens driving mechanism which moves a lens. Therefore, the camera modules have disadvantages such that they are complicated in structure and that they need relatively large amount of electric power. Furthermore, the camera modules have a disadvantage such that they are generally low in impact resistance.
In view of this, various focus adjusting mechanisms which do not require a lens driving mechanism that moves a lens have been suggested for the purpose of downsizing the camera modules and reducing electric power consumed by the camera modules. Such focus adjusting mechanisms are disclosed in, for example, Patent Literature 1 through 3.
A focus adjusting mechanism disclosed in Patent Literature 1 employs a method of adjusting a focus by changing a refractive index of a liquid crystal lens. According to the method, a liquid crystal lens is provided. By changing refractive index distribution of the liquid crystal lens by changing a voltage (drive voltage) applied to the liquid crystal lens, the liquid crystal lens is caused to function as a varifocal lens.
A focus adjusting mechanism disclosed in Patent Literature 2 is configured such that a drive shaft is connected to a piezoelectric element and is provided with a lens. According to the focus adjusting mechanism, the piezoelectric element is expanded and contracted by applying a voltage to the piezoelectric element. This causes the drive shaft to be moved and, accordingly, causes the lens to be moved.
A focus adjusting mechanism disclosed in Patent Literature 3 employs a liquid lens.
A focus adjusting mechanism disclosed in Patent Literature 4 includes a polymer-type lens. A cover is provided on a soft polymer layer, and a piezoelectric element is provided on the cover. According to the focus adjusting mechanism, the piezoelectric element is expanded and contracted by applying a voltage to the piezoelectric element. This causes the cover and the soft polymer layer to be deformed and, accordingly, causes a focal distance to be adjusted.
Such a focus adjusting mechanism that does not require a lens moving mechanism is generally mounted on an upper surface of a lens barrel of a fixed focus camera module. The focus adjusting mechanism causes the fixed focus camera module to function as a varifocal camera module. Specifically, in a case where a voltage is applied to the focus adjusting mechanism, this causes a change in a refractive index and, accordingly, causes a focal position to be adjusted.
A step of assembling the fixed focus camera module includes a step of adjusting a height of the lens barrel including the lens. This step is generally carried out so as to adjust a focus of the camera module to infinity (for example, 2 m to 4 m).
As a method of carrying out the step of adjusting a height of a lens barrel, there is kwon a method in which a height of a lens barrel is adjusted by (i) forming threads on an outer surface of the lens barrel and an inner surface of a holder of a camera module which threads engage with each other and (ii) rotating the lens barrel in the holder.